Process for manufacturing products from acetylated wood fibre

ABSTRACT

A process for forming wood fibre comprising breaking down acetylated wood to produce acetylated wood fibre having a moisture content from about 5% w/w to about 8.5% w/w after it is comminuted to form wood fibre. The process includes a first moisture-introducing step for increasing the moisture content of the acetylated wood elements; and a second moisture-introducing step, separate to the first, for increasing the moisture content of the acetylated wood dements.

FIELD OF THE INVENTION

This invention relates to the field of wood processing. Morespecifically, it relates to processing acetylated wood fibre for use inmanufacturing of products from the acetylated wood material includingwood board manufacture such as wood fibreboard manufacturing.

BACKGROUND TO THE INVENTION

Boards constructed from wood such as wood fibreboards, for examplemedium density fibreboard (MDF) comprising wood fibres bound togetherwith binder resin are superior in strength and are easily processed dueto their homogeneity. Such wooden products including fibreboards can beused to obtain a variety of formed shapes. The shapes can be planar orflat in profile. Curved shapes are also easily formed. They are widelyused as materials for furniture and for a variety of constructionpurposes.

In resin fibreboards such as MDI (methylene diphenyl diisocyanate) resintype fibreboards such as MDF, in which the wood fibres are bondedtogether by means of MDI; the dimensional stability variations due tothe hygroscopicity (water retaining property) of the material and waterabsorption are great. The fibreboard such as MDF can also be subject tobiological decay.

Chemical modification of wood for improved dimensional stability andbiological decay protection has been the subject of research for manyyears. Acetylation is one such method that has been well researched anddocumented. In the acetylation process, in order to make effective useof expensive acetic anhydride and to prevent it reacting with moisturein the wood, the wood may be dried to obtain a low moisture content,typically a content of less than about 3%. During acetylation, thechemical reaction of the acetic anhydride substitutes the hydroxylgroups in the wood cells with acetyl groups. This has the effect ofbulking out the walls of the wood cells and preventing moisture uptake,and hence gives the treated wood a level of hydrophobicity (resistanceto water intake) and dimensional stability much greater than that ofnon-acetylated wood. The resulting acetylated wood has low moisturecontent and enhanced resistance to biological decay.

Unprocessed green wood may have a moisture content of greater than 50%.When manufacturing wood fibreboard such as MDF, the manufacturingprocess normally involves reducing the moisture content of the greenwood by mechanical compressing of wood chips and subsequent softening ofthe wood chip by heating with steam. This aids the extraction of thefibre for further processing into wood fibreboard. As wood is anexcellent insulator of heat, the wood chip requires a high moisturecontent to allow an efficient thermal heat transfer into its core toenable the softening of the chip to take place.

A thermo-mechanical defibration or refining process is generallyperformed to breakdown the softened chip into fibre. The results of theprocess are dependent on variations in the moisture content of the wood,the heat applied to the wood chip and the point at which the woodconstituents enters their glass transition phases (i.e. the transitionfrom a hard and relatively brittle state into a softened or rubber likestate).

Further conversion of the fibres into MDF is influenced by the fibrequality, density and moisture content.

In acetylated wood fibreboard manufacturing, the acetylated wood chip islow in moisture content (approx. 7% moisture content) has a higherdensity that non-acetylated wood chip and exhibits high levels ofhydrophobicity. Unlike MDF processing using non-acetylated wood,processing of such a dry wood chip requires the introduction of moistureinto the chip rather than a reduction of moisture in the chip.Furthermore, the temperature/moisture conditions necessary to achievethe glass transition conditions for the thermo mechanical conversionchip into fibre must be established.

Moisture plays an integral part in the composition of wood fibre in themanufacture of MDF panels and other products. The moisture in the fibreperforms a number of functions. It enables even heat distribution to beachieved in the forming press across the fibres, and is necessary toinitiate the chemical bonding action with bonding resins used tofabricate wood products. Thus, producing a wood fibre within a desiredmoisture range which allowed for the production of acetylated and nonacetylated wood products on the same production line would providesignificant advantages.

Internal voids are formed in an MDF fibreboard when the heat produced inthe forming press causes the moisture to evaporate at a rate in theforming process which precludes its escape through the surface of theboard. Surface blemishes occur in the forming press when the moistureescapes through the surface of the board after the board has left theforming press. The primary factor in the formation of these defectsarises from the actual moisture content of the wood fibre at the startof the process. Other factors which influence the formation of thedefects lie in the temperature, speed and pressure conditionsexperienced by the panel in the forming press. While these can influencethe formation of the defects, moisture content is the major contributor.

As such, the controlled introduction of moisture is of criticalimportance in the fibreboard manufacturing process and improvements inthe methods of moisture introduction would lead to higher quality woodfibres with resultant benefits found in products made from those fibres.

In MDF wood fibreboard manufacturing using non-acetylated wood, there isa risk of explosion associated with the possible ignition of a dry wooddust atmosphere. Much of this risk is mitigated by the relatively highmoisture content state of the wood chips. However, in acetylated woodfibreboard manufacturing the risk of explosion is much greater due tothe low moisture content of the wood elements, the high levels ofhydrophobicity of the wood element and the associated generation oflarge amounts of dry dust. This is a significant issue to be overcome.

The capital cost of a plant in MDF wood fibreboard manufacturing is veryhigh. In order to commercialise acetylated wood fibreboardmanufacturing, the acetylated wood elements should ideally be processedon the same plant as the non-acetylated wood fibreboard. Yet thedifferences in characteristics of non-acetylated and acetylated woodelements are significant and present particular challenges in gettingthe processing equipment to function effectively for both sets of woodelements. Changes in the processing techniques used to successfullyprocess non-acetylated wood elements would be required to successfullyprocess acetylated wood elements. The changes must be activatable anddeactivable or reversible, as the plant must accommodate both sets ofwood elements.

There is a need for improvements to the process of manufacturing woodfibreboard comprising wood fibres bound together with a binder resinthat will allow acetylated wood fibreboard to be manufactured on thesame plant as non-acetylated wood fibreboard.

Much of the prior art to date has been concerned with the acetylationprocess of the wood elements and whilst laboratory scale production ofacetylated fibreboard has taken place there is little literatureavailable on overcoming the difficulties in upscaling the laboratoryproduction into full commercial production onto existing MDF plants.There is an absence of guidance on how to commercially condition thewood chip, how to avoid explosive risks, and little guidance on how topress the acetylated fibres into uniform panel thicknesses.

In the publication “A NEW PROCESS FOR THE CONTINUOUS ACETYLATION OFLIGNOCELLULOSIC FIBRE” by Rune Simonson and Roger M. Rowell, it is notedthat the wood element is converted to fibre in green state prior toacetylation. Post acetylation, the acetylated fibre can be resinated forfibreboard production. There exists no further detail on how the processis executed. In U.S. Pat. No. 6,376,582, “WOOD FIBREBOARD ANDMANUFACTURING METHOD THEREFOR,” reference is made to the manufacture ofacetylated MDF using a mixture of acetylated and non-acetylatedmaterials. No guidance is provided as to how the fibres are formed forMDF panel processing.

U.S. Pat. No. 6,632,326 MODIFYING METHODS FOR WOOD ELEMENTS discloses aprocess of acetylating wood by subjecting it to a gaseous acetylatingagent. The wood elements are then digested for 2-5 minutes underhigh-pressure steam at a temperature of around 150-170° C. Wood fibresare obtained by separation of the wood elements into fibres through adisk refiner. The experimental section is based on laboratory scale workand provides no teaching on how to handle materials at a commercialscale. Furthermore the description provides little guidance on thehazards in processing excessively dry fibre. WO 2011/095824 A1 PROCESSFOR THE ACETYLATION OF WOOD ELEMENTS discloses a process of breakingdown acetylated chip to fibre by passing through a conventionaldefibrator, combining with pMDI adhesive, and converting to compositepanel or board by applying high temperature and pressure. No guidance isprovided as to how the fibres are formed for MDF panel processing. Thedescription provides no guidance on the hazards in processingexcessively dry fibre.

SUMMARY OF THE INVENTION

The present invention provides for a process for forming wood fibrecomprising breaking down acetylated wood to produce acetylated woodfibre having a moisture content from about 5% w/w to about 8.5% w/wafter it is comminuted to form wood fibre.

The process provides the advantage of producing an acetylated wood fibrecomprising a moisture content which results in enhanced strength andwater resistance in wood products fabricated from the fibre.Furthermore, the process may be carried out in plants using machinerywhich is also suitable for the production of non-acetylated wood fibre.

Unlike prior art fibre where acetylated and non-acetylated wood fibre ismixed, the present invention does not include wood fibre where fibrefrom a non-acetylated source is mixed in with the acetylated wood fibre.The acetylated wood fibre has this moisture content and is suitable forbeing combined with a binding material such as resin to form a productsuch as a fibreboard.

The wood fibre of the process may be formed from breaking down pieces ofwood such as wood chip.

The wood fibre may have a moisture content of about 5% w/w to about 8%w/w after it is comminuted to form wood fibre, for example from about5.5 to about 7.5% w/w.

The wood fibre may have a moisture content of from about 6% to about 7%w/w after it is comminuted to form wood fibre. Desirably the acetylatedwood fibre has a moisture content from about 6.5% to about 6.8% w/w.

The moisture content of the acetylated fibre makes it safer forhandling. Furthermore products formed from the acetylated fibre are muchless likely to suffer defects such as blistering and/or delamination.

Moisture plays an integral part in the composition of wood fibre in themanufacture of fibreboards such as MDF panels. The moisture in the fibreperforms a number of functions. It enables even heat distribution to beachieved in the forming press across the fibres, and is necessary toinitiate the chemical bonding action with the resin such as MDI resin.

Internal voids are formed in fibreboards such as an MDF panel when theheat applied in the forming press causes the moisture to evaporate at arate in the forming process which precludes its escape through thesurface of the product such as a board. Surface blemishes can thus occurin the product when the moisture escapes through the surface of theproduct after the product has left the forming press. The primary factorin the formation of these defects arises from the actual moisturecontent of the wood fibre as introduced into the forming press and thusat the start of the forming part of the process. Other factors whichinfluence the formation of the defects lie in the temperature, speed andpressure conditions experienced by the panel in the forming press. Whilethese can influence the formation of the defects, moisture content isthe major contributor. Surface blemishes and internal voids can alsooccur through insufficient moisture. The mechanism causing the defectslies in the lack of moisture needed to initiate the resin (e.g. MDI)chemical bonding action or curing process occurring in the formingpress.

Empirical data indicates that a fibre moisture content of 12% increasesthe incidence of internal voids and surface blemishes being formed inthe pressing process in the manufacture of MDF to 98% to 100% of totalproduction.

Within the process, the moisture content of the acetylated wood may beadjusted by adding moisture in more than one processing step.

The process may comprise a first moisture-introducing step forincreasing the moisture content of the acetylated wood elements; and asecond moisture-introducing step, separate to the first, for increasingthe moisture content of the acetylated wood elements.

Introducing the moisture in this manner reduces the risk of explosionassociated with processing of low moisture content wood elements such anacetylated wood. It further provides the advantage of preventingover-moisturising of the chip which will reduce the quality of the fibreproduced and will thus have a further negative impact on the strengthand durability characteristics of products produced from the fibre.Over-moisturising may even render the chip unsuitable for furtherprocessing.

The moisture may be introduced to the process in the form of water andin the form of steam.

A first moisture-introducing step may comprise introducing water, andintroducing steam in order to heat the chip.

Steam may be introduced to the process at a temperature in the range ofabout 160° C. to about 190° C.

Steam may be introduced to the process at a temperature in the range ofabout 175° C. to about 185° C.

Steam may be to the process introduced at a temperature of about 180° C.

The second moisture-introducing step of the process may compriseintroducing steam in order to heat the chip.

The second moisture-introducing step of the process may compriseintroducing steam at a temperature in the range of about 170° C. toabout 210° C.

The second moisture-introducing step of the process may compriseintroducing steam at a temperature in the range of about 180° C. toabout 200° C.

The second moisture-introducing step of the process may compriseintroducing steam at a temperature of about 190° C.

The acetylated wood of the process may be in the form of wood piecessuch as wood chip and a first moisture-introducing step is performed ina receptacle containing the pieces of acetylated wood.

The receptacle may be a holding receptacle for feeding the acetylatedwood for further processing.

The receptacle may be a surge control receptacle such as a surge bin forconsistent feeding of the acetylated wood for further processing.

This provides the advantage of a continuous flow of material though theprocess.

Inconsistent feeding of wood elements in the processing line may damageequipment. Lumped or bulky feeding can lead to clogging of equipmentwhile feeding wood elements too thinly can also damage equipment.

In the process, the acetylated wood may be treated in a digester andmoisture is added to the acetylated wood in the digester.

Moisture may be added to the acetylated wood in the digester in a secondmoisture-introducing step.

Each of the above steps allow for the controlled introduction ofmoisture which is of critical importance in the fibreboard manufacturingprocess. Controlled moisture introduction leads to higher quality woodfibres with resultant benefits, such as enhanced strength and durabilityfound in products made from those fibres.

The invention further provides for a process for forming an article froman acetylated wood fibre formed by the process as described above. Thewood fibre may be formed into an article by bonding the wood fibre usinga suitable bonding agent such as a resin. The article may be a suitablefibreboard.

The invention further provides for a product such as a fibreboardproduced from the process of the invention.

The acetylated wood may be passed through a compressive screw feedersuch as a plug screw feeder before being treated in a digester.

The compressive screw feeder may be suitable for handling bothacetylated and non-acetylated wood pieces, the screw feeder comprising:a housing for a screw element, the housing having an intake and anoutlet, a rotatable screw element for rotating to compressively progressthe wood pieces through the screw feeder from the intake to the outlet;wherein proximate the outlet, a compacting end of the screw isdimensioned to allow compressive progress of acetylated wood pieces sothat acetylated wood pieces having a moisture content from about 3% toabout 10% w/w exit the compressive screw feeder.

The screw element of the screw feeder may be adapted by changing thepitch of the screw element at the outlet end thereof so that the screwelement has two different pitches.

Moisture may be added to the wood as it is fed to the screw feederand/or as it passes through the screw feeder.

The acetylated wood may be passed through a screw feeder after beingtreated in a digester and before being refined in a refiner.

Moisture may be added to the wood as it is fed to the screw feederand/or as it passes through the screw feeder.

The refining step may be performed by refining the acetylated woodbetween two discs comprising a fixed disc and a rotating disc separatedby a gap.

The refining step may be performed in a pressurised chamber.

The present invention further provides for a process for forming anarticle from acetylated wood fibre, the process comprising takingacetylated wood fibre as described above or an acetylated wood fibreformed by the process as described above and forming the wood fibre intoan article by bonding the wood fibre using a suitable bonding agent suchas a resin.

The process may be for forming a suitable fibreboard.

The acetylated wood fibre may be formed by the process described aboveand may further comprise a drying step after the refining step.

A binder resin may be injected into the wood fibre during the dryingstep. The drying step may comprise passing the fibre through one or moreheaters comprising an inlet and an outlet.

In the present invention the term “wood fibre(s)” as applied tomaterials for use in the production of articles made from wood fibre,does not include fibres naturally bound together within a piece of wood,instead it refers to the material obtained when wood has been brokendown (by processing) into particulate matter. It can be consideredcomminuted wood material. Of particular interest in the presentinvention is particulate matter which is fibrous in nature and of a typesuitable for use in the manufacture of a fibreboard such as MDF.

In the present invention reference is made to water and to steam. Itwill be appreciated by the skilled person that the two terms are used todifferentiate between water (whether heated or not) in liquid form andwater in its gaseous form. For example where water and steam are addedit is clear that this means that liquid water and gaseous steam are bothadded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram of the process up the refiner stageincluding the moisture adding stages

FIG. 2 shows a flow diagram of the heating stages of the process

DETAILED DESCRIPTION

The present invention will now be described with reference to theaccompanying drawings.

FIG. 1 shows a typical plant set up suitable for the processing of woodfibreboard into fibreboard and for the formation of MDF from the fibre.The process has been modified to allow for the processing of acetylatedwood chips into fibre and the formation of fibreboards from the fibre.The overall process for the formation of the fibreboards will bedescribed below and the modifications from the typical MDF process toallow for the processing of acetylated wood elements will behighlighted.

Acetylated wood material is cut into chips 1 and is held in storage 2until required for processing.

A variety of acetylated wood materials may be used. However, preferredsources for the starting material include aspen, radiata pine,lodge-pole pine, Japanese cedar, Japanese cypress, larch, white fir andspruce.

Acetylated wood pieces or elements are collected by an infeed hopper 3and are conveyed along an infeed conveyor 4 into a surge bin 5.Preferably, elements are fed into the bin at a rate of about 20 m³/hr.Preferably, the average size of the wood elements at this stage is about25 mm×25 mm×6 mm. In order to sustain such a rate, the surge bin 5preferably has a capacity of about 7 m³. The surge bin 5 has the effectof changing an inconsistent flow of material into a controlled flow or aconsistent flow.

Water is added to the chip 1 in the surge bin 5. Preferably, a mainswater supply of about 10 l/min is added as chip enters the bin. Watermay be added through a series of nozzles. In a preferred embodiment,water is added through four nozzles into the top of the surge bin. Thishas the effect of adding surface moisture and reducing the generation ofdust. This is additional step compared to the processing ofnon-acetylated wood. Such a step is not necessary in non-acetylated woodelement processing due to the higher moisture content of thenon-acetylated wood. Steam at a pressure of 9 bar (0.9 MPa) is injectedto the base of the surge bin at a rate preferably in a range from 1900kg/h to 2200 kg/h, for example at about 2099 kg per hour. In a preferredembodiment, steam is added at a temperature of about 180° C. The steammay be added through nozzles at the base of the bin. In a preferredembodiment, steam is added through three nozzles positioned at 120° C.at about 300 mm from base of bin.

The throughput of chips is dependent on line speed at the board formingend of the process. i.e. it is dependent on how much volume of board perhour is to be produced. If the line speed is running at 10 m³ per hourthen the throughput of chips will be 10 m³ per hour.

Steam is preferably added at a fairly constant rate. The line speed ispreferably 10 m³ per, with about a 10% variance. The rate of addition ofsteam would not be adjusted for this variance. This has the effect ofheating the chip. The chip is heated in order to soften it.

Because of the low moisture content of acetylated chip, the time takento heat the chip is longer compared to non-acetylated chip. As such, thesteaming or heating step may be split and carried out in two locations.The first heating step may take place in the surge bin 5 as describedabove where the chip is heated to about 100° C. and the second occurslater in the process, in a digester 8 (as described below). This allowsadditional time to be added to the heating process without affectingplant throughput. In an embodiment with a 5 m³ running capacity of thebin and a 10 m³ per hour line speed, the chip will remain in the surgebin for about 30 minutes before being passed out of the bin.

After being partially heated in the surge bin 5, the acetylated woodchip 1 passes through a chute 22. In a preferred embodiment, thedimensions of chute are about 600 mm×600 mm. Water is added the chute atrate preferably in a range from 20 l/min to 30 1/ min, for example atabout 25 l/min to the chip 1. (The water is at the temperature of amains water supply. At this point in the process, the volume of chip isline speed dependent, the size of the chip is the same as when it hasleft the surge bin as there has been no mechanical change other than toheat the chip). The water is added using two injection points on thechute 22. In a preferred embodiment, water is added from injectionpoints at 500 mm from the base of the chute. This is an additional stepcompared to the processing of non-acetylated wood. A window is added tothe area of the surge bin 5 which contains the partially steamed chip.Viewing the chip through this window allows the level of chip insertedto be monitored and verified. This aids process control by allowingblockages with the wetted, heated acetylated chip to be detected.

From the surge bin 5, the acetylated wood chip 1 passes through amodified plug screw feeder 6. The plug screw feeder is modified in thesense that it differs from a plug screw feeder normally used for thefeeding of non-acetylated wood. The compacting area 7 of the screw isreduced compared to that of a feeder normally used with non-acetylatedwood. This allows the acetylated wood chip to pass through this sectionof the process. The purpose of the plug screw in normal MDFmanufacturing is to remove some of the moisture from the chip, to feedthe chip into the digester 8, and to maintain a seal in the digester 8which is at 9 bar (0.9 MPa) pressure. Acetylated chips are harder andmuch drier than non-acetylated chips, (starting moisture content ofacetylated chip in the screw feeder is approx. 12% w/w whilenon-acetylated chips >50% w/w). The acetylated chip has less moistureand is more brittle and has a higher density than non-acetylated wood. Acompromise is needed in the screw design to allow it work over this widerange of physical conditions of the chip (i.e. the allow the same plugscrew to work with the dryer denser harder acetylated chip and thewetter, softer, lighter non acetylated chip). Reducing the compressionat the end of the screw, while maintaining its compressioncharacteristics over its length, allows for the final passage ofacetylated wood through the screw feeder, and also allows effectivecompression of non acetylated wood elements when these are beingprocessed at other times. The compromise design, i.e., the reduction incompression at the end of the screw can be achieved in accordance to theratio L×0.1759 and D×0.734. For example a non acetylated wood screw oflength L=2245 mm and whose end diameter D=150 mm over a length of 250 mmwill have a new reduced end diameter D=110 over a length of 395 mm in anadaptation to an acetylated screw feeder. In a preferred embodiment, theoverall dimensions of the plug screw feeder are about 3900 mm×420 mm.The plug feeder load is from 25-35% of maximum load, for example theplug feeder load is about 30% of its maximum load. In a preferredembodiment, the plug screw feeder is equipped with a 338 kW motor. Thethroughput of the plug screw feeder is limited to the line speed of theprocess. The maximum throughput of the plug screw feeder is 20 m³ perhour on non acetylated chips. The maximum throughput, for acetylatedchip is limited to line speed which is about 10 m³ per hour. The plugfeeder speed is preferably from 15% to 25% of its maximum, for example,the plug feeder speed is about 20% of maximum.

The acetylated wood chip 1 passes from the screw feeder 6 into adigester 8. The digester further softens the wood elements. The woodchip 1 is retained in the digester for 6 minutes. Steam at about 9 bar(0.9 MPa) is injected. In a preferred embodiment, steam is injectedthrough four nozzles. Two nozzles are located on either side of thedigester. In a preferred embodiment, steam at 190° C. is injected viacontinuous injection into the digester at a rate preferably in a rangefrom 2750 to 3048 kg/h. For example steam is injected at a rate of about2898 kg per hour. The temperature in the digester is maintained at about182° C. This rate of steam injection is lower compared withnon-acetylated wood element processing, where a rate of 5000 kg per hourmay be expected.

Wood elements are discharged from the digester by way of internaldischarge screw 9 at a rate dependent on the line speed. A-rotatingaction of the discharge screw transports the wood elements onto adefibrator feeder ribbon screw 23. Water is injected into the feederribbon screw 23 housing at a rate preferably in the range of 25 l/min to35 l/min, for example water is injected at a rate of 30 l/min. In apreferred embodiment, water is injected through two nozzles on eitherside of the screw at about 450 mm from the entry point to the screw. Thewater injection is continuous for as long as the line is running. Theinjected water has a temperature preferably in a range of 50° C. to 98°C., for example at a temperature of 90° C. The chip 1 is feed via thefeeder ribbon screw 23 into a low energy plate refiner 24.

The addition of the hot water into the refiner in the manner describedabove is necessary to achieve the correct balance between moisturecontent and heat content of the acetylated chip in order to maintain theglass transition phase of the wood element. By contrast, no injection ofwater is necessary in the processing of non-acetylated wood elements asthe chip arrives at the refiner 24 with the correct combination ofmoisture/temperature to maintain the glass transition state. The glasstransition point is not a fixed defined point. The process deals with anatural wood which by its nature is not uniform in its makeup andconsistency. So the quality of the fibre produced in the refiner willindicate that the glass transition point has been achieved.

After being fed by the ribbon screw feeder 23 into the refiner 24, thechip 1 is retained in the refiner 24. The rate of feeding into therefiner is again line speed dependent. A differential pressure in arange preferably from 0.0063 to 0.0103 bar (630 Pa-1030 Pa) ismaintained in the refiner, for example a differential pressure of about0.0083 bar (830 Pa) is maintained. The refiner blow valve position isheld at 22% of full opening. The incoming pressure into the refinercomes from the pressure in the digester at about 9 bar (0.9 MPa). Assuch, the refiner is essentially at a 9 bar pressure. The blow valve isused to regulate the flow of material through the refiner. Thedifferential pressure is a means of controlling this flow. For examplefor wood fibreboard, with the blow valve opened at 56% of full opening,not a lot of back pressure is maintained as the material exits therefiner, so the difference in incoming pressure from the digester andthe refiner pressure, i.e. the differential pressure, is 0.1712 bar (17KPa) (9 bar (0.9 MPa) in the digester and 8.8288 bar (0.882 MPa) in theactual refiner).

For acetylated wood, the blow valve is shut down to 22% of full opening.This results in the incoming pressure from the digester being maintainedin the refiner, hence the ‘differential pressure’ is reduced to 0.0083bar (830 Pa). This differential pressure measurement indicates aretention time within the refiner which correlates to an amount of timethe wood is in the refiner being transformed into fibre. If it is in therefiner for too long a time, the wood fibre turns to fines or dust whichcannot be used in mdf. If it is in the refiner for too short a time itis not refined enough and shives are produced which cannot be used tomake mdf. In a preferred embodiment, the refiner consists of two discs,one fixed, the other rotating at about 1490 rpm. The discs have a gap ofabout 14 mm. Chip slurry enters through the centre of the refinersimilar to a centrifugal pump and travels to outside of the disc. Duringthis route it gets rubbed and cut which changes the chip into a fibre.The diameter of the discs are approx 1.5 m, so for a 10 m3 per hour linespeed and a rpm of 1490, a gap of 14 mm, the retention time in therefiner will be about 0.115 secs. Again the retention time is fine tunedby the plate gap, and a discharge valve. What ultimately drives the finetuning is the ‘quality’ of fibre on the forming line. Again, the processdeals with a natural chip which is inconsistent, hence the range ofparameters. The fibre size is measured by sieving the wood elementsthrough meshes with different opening sizes. As such, ‘good quality’board could be expected to be produced with approx. the following fibresizes: 0%>4 mm mesh size, 2% >2 mm, 8%>1.25 mm, 15%>0.8 mm, 18%>0.5 mm,20%>0.25 mm, 20%>0.125 mm and 17%<0.125 mm.

Other refiner 24 operating conditions may be set as follows: the refinerfeed screw speed preferably operates in a range from 28 to 48% ofmaximum for example the feed screw speed may be about 38% of maximum.The refiner feed screw load is preferably in a range from 18 to 28% ofmaximum, for example the feed screw load may be about 23% of maximum.The feed screw speed is not a set condition, as it is line speeddependent eg if line speed drops to 5 m³ p/h the feed screw will alsodrop to this amount. The refiner plate position is preferably in a rangefrom 13 mm to 17 mm, for example the refiner plate position may be about15 mm. The chamber hydraulic pressure is preferably in a range from 10to 14 bar (0.1 to 1.4 MPa), for example, the pressure may be about 12bar (1.2 MPa). The refiner main drive power is preferably in a rangefrom 539 kw/h to 739 kw/h, for example, the drive power may be 639kw/h.In a preferred embodiment, the refiner motor is sized to be about 3150kw. The refiner efficiency is preferably in a range from 79 kW/t to99kW/t, for example the efficiency may be 89 kW/t.

The fibres from the refiner are passed through a blow valve 10 andfurther through a blow line to a stage 1 dryer 12 (FIG. 2). The rate ofpassage or volume of fibre passed is line speed dependent. In apreferred embodiment, the diameter of the blow line is about 100 mm. Theacetylated wood fibres 1 from the refiner 8 are coated with a binderresin such as methylene diphenyl diisocyanate (MDI). The binder resin at6% w/w is injected into the blow line 10 from the refiner 24 to theStage 1 Dryer 12. In a preferred embodiment, the resin is injected in acontinuous flow via a single point injection. The injection in thepreferred embodiment is through a nozzle about 1.5 m from the exit ofrefiner. In addition, curing agents, curing catalysts, curingaccelerators, diluents, thickeners, adhesive compounds, dispersingagents, and water repelling agents may be added to the binder resin asneeded.

By comparison, for a wood fibreboard process, binder resin is injectedinto the blow line at 4% w/w. In order to prevent the moisture in theacetylated fibre reducing to dangerous levels and thus risking anexplosion, flue gas dampers may be closed on the stage 1 dryer 12. Astack on dryer stage 1 may also be opened fully to reduce thetemperatures in the dryer.

The Stage 1 Dryer 12 inlet temperature is controlled preferably at atemperature in a range from 84° C. to 104° C., for example thetemperature may be controlled at about 94° C. The Stage 1 outlettemperature is controlled preferably at a temperature in a range from45° C. to 65° C., for example the temperature may be controlled at about55° C. The moisture content of the acetylated fibre exiting the Stage 1Dryer 12 is about 11% w/w. The fibre is dried until its moisture contentis measured to be about 11% w/w.

The acetylated fibre continues from the stage 1 dryer 12 to Stage 2Dryer 13. The Stage 2 Dryer 13 inlet temperature is controlledpreferably at a temperature in a range from 52° C. to 72° C., forexample the temperature may be controlled at about 62° C. The Stage 2dryer 13 outlet temperature is controlled preferably at a temperature ina range from 28° C. to 48° C., for example the temperature may becontrolled at about 38° C. The moisture content of the acetylated fibreexiting the Stage 2 Dryer 13 is about 8% w/w. The fibre is dried untilits moisture content is measured to be about 8% w/w.

Experimental

Moisture plays an integral part in the composition of wood fibre in themanufacture of fibreboards such as MDF panels. The moisture in the fibreperforms a number of functions. It enables even heat distribution to beachieved in the forming press across the fibres, and is necessary toinitiate the chemical bonding action with the MDI resin. Internal voidsare formed in an MDF panel when the heat produced in the forming presscauses the moisture to evaporate at a rate in the forming process whichprecludes its escape through the surface of the board. Surface blemishesoccur in the forming press when the moisture escapes through the surfaceof the board after the board has left the forming press. The primaryfactor in the formation of these defects arises from the actual moisturecontent of the wood fibre at the start of the process. Other factorswhich influence the formation of the defects lie in the temperature,speed and pressure conditions experienced by the panel in the formingpress. While these can influence the formation of the defects, moisturecontent is the major contributor. Surface blemishes and internal voidscan also occur through insufficient moisture. The mechanism causing thedefects lies in the lack of moisture needed to initiate the MDI chemicalbonding action or curing process occurring in the forming press.

Empirical data indicates that a fibre moisture content of 12% increasesthe incidence of internal voids and surface blemishes being formed inthe pressing process in the manufacture of MDF to 98% to 100% of totalproduction.

Typical ingredient profile of the fibre entering the forming press wouldbe:

Wood Fibre 80%, MDI resin 6%, Release Wax 2%, moisture 12%

Acetylated wood fibre has inherent characteristics which differentiateits reactive performance under wood fibre processing conditions. Itsmolecular structure has been altered by the substitution of some of thehydroxyl groups with acetyl groups, this substitution imparts a degreeof hydrophobicity to the acetylated fibre elements. As a consequence ofthe molecular substitution the density of the fibre increases by about20%.

Testing of the effects of different levels of moisture on defects levelswere carried out according to the parameters below. In collating theresults, each panel in 1220×2440 mm size format were passed under theImal Ultrasonic detector. Any defect acknowledged by the detector wouldcause the board to be rejected. The results tabulated are percentrejected boards during each different fibre moisture set points test.

Fibre Composition

Acetylated Wood Fibre 80%, MDI resin 6%, Release Wax 2%, moisture varied

Forming press temperature 180° C.,

Forming Press pressure 18.7 kgf/cm²

Defect Detection: Imal Ultrasonic blow blister detector.

The Forming Press Profile for each of the sample boards is shown inFigure A below.

Results:

Table I shows the percentage number boards rejected due to defects for anumber of board thicknesses (6 to 18 mm) over for range of fibremoistures (3% to 12%).

For example, for a 9 mm thick board with 8% moisture content, 4.4% ofboards are rejected.

TABLE I 12% 9% 8% 7% 6% 5% 4% 3% 18 mm 98% 52% 4.29 1.2 1.1 4.51 48.1996.9 12 mm 95% 54% 4.0% 1.1% 1.0% 4.8% 50% 97%  9 mm 97% 50% 4.4% 1.2%1.1% 4.6% 48% 96%  6 mm 100% 52% 4.1% 1.3% 1.09%  4.5% 47% 96%

Table II shows the percentage number boards rejected due to defects forboard thickness of 12 mm over for range of fibre moistures (5.5% to7.5%).

For example, for a 12 mm thick board with 6.5% moisture content, 0.091%of boards are rejected.

TABLE II 7.5% 7 6.8% 6.5% 6% 5.5% 12 mm 1.9% 1.17% 0.2% 0.091% 1.1% 2.0%

Conclusions

The first series of tests at different moisture levels on 4 differentthicknesses of boards confirmed that the thickness of the board hadlittle influence on the formation of defects within the board. Furthertesting at different moisture levels were thus confined to onethickness. Test data generated from this series of tests at differentmoisture levels suggest that at moisture level of 5.5% to 7.5%, thepercentage reject panels decrease to 2%, while a moisture level in therange 6% to 7% will half this level of rejects. Ideally a range of 6.5%to 6.8% will minimise the formation of defects.

The words “comprises/comprising” and the words “having/including” whenused herein with reference to the present invention are used to specifythe presence of stated features, integers, steps or components but donot preclude the presence or addition of one or more other features,integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable sub-combination.

1. A process for forming wood fibre comprising: breaking down acetylatedwood to produce acetylated wood fibre having a moisture content fromabout 5% w/w to about 8.5% w/w after it is comminuted to form a woodfibre.
 2. The process according to claim 1 wherein the wood fibre isformed from breaking down pieces of wood.
 3. The process of claim 1wherein the wood fibre has a moisture content of about 5% w/w to about8% w/w after it is comminuted to form wood fibre.
 4. The process ofclaim 1 wherein the wood fibre has a moisture content from about 5.5% toabout 7.5% w/w.
 5. The process of claim 1 wherein the wood fibre has amoisture content of about 6.5% to about 6.8% w/w after it is comminutedto form the wood fibre.
 6. The process of claim 1 wherein the moisturecontent of the acetylated wood is adjusted by adding moisture in morethan one processing step.
 7. The process of claim 1 further comprising:a first moisture-introducing step for increasing the moisture content ofthe acetylated wood elements; and a second moisture-introducing step,separate to the first, for increasing the moisture content of theacetylated wood elements.
 8. The process of claim 1 wherein the processfurther comprises: introducing moisture is introduced in the form ofwater and in the form of steam during the process.
 9. The process ofclaim 1 wherein the process further comprises: a firstmoisture-introducing step that comprises introducing water, andintroducing steam in order to heat the acetylated wood.
 10. The processof claim 1 wherein the process further comprises: introducing steam at atemperature in the range of about 160° C. to about 190° C.
 11. Theprocess of claim 1 wherein the process further comprises: introducingsteam is introduced at a temperature in the range of about 175° C. toabout 185° C.
 12. The process of claim 1 wherein the process furthercomprises: introducing steam at a temperature of about 180° C.
 13. Theprocess of claim 7 wherein the second moisture-introducing stepcomprises introducing steam in order to heat the acetylated wood. 14.The process claim 7 wherein the second moisture-introducing stepcomprises introducing steam at a temperature in the range of about 170°C. to about 210° C.
 15. The process of claim 7 wherein the secondmoisture-introducing step comprises introducing steam at a temperaturein the range of about 180° C. to about 200° C.
 16. The process of claim7 wherein the second moisture-introducing step comprises introducingsteam at a temperature of about 190° C.
 17. The process of claim 1wherein the acetylated wood is in the form of wood pieces and saidprocess further comprises a first moisture-introducing step that isperformed in a receptacle containing the pieces of acetylated wood. 18.The process of claim 17 wherein the receptacle is a holding receptaclefor feeding the acetylated wood for further processing.
 19. The processof claim 18 wherein the receptacle is a surge control receptacle forconsistent feeding of the acetylated wood for further processing. 20.The process of claim 1 wherein the acetylated wood is treated in adigester and moisture is added to the acetylated wood in the digester.21. The process of claim 20 wherein moisture is added to the acetylatedwood in the digester in a moisture-introducing step.
 22. A process forforming an article from an acetylated wood fibre, said processcomprising: breaking down acetylated wood to produce acetylated woodfibre having a moisture content from about 5 w/w to about 8.5% w/w afterit is comminuted to form a wood fibre, and forming the wood fibre intoan article by bonding the wood fibre using a bonding agent.
 23. Theprocess according to claim 22 wherein the article is a fibreboard.
 24. Aproduct produced according to the steps: breaking down acetylated woodto produce acetylated wood fibre having a moisture content from about 5%w/w to about 8.5% w/w after it is comminuted to form a wood fibre, andforming the wood fibre into an article by bonding the wood fibre using abonding agent.
 24. The process of claim 2, wherein said pieces of woodare wood chip.
 25. The process of claim 22, wherein said bonding agentis a resin.